Over the last decade or so, the utility of inorganic nanoparticles as additives to enhance polymer performance has been well established. Ever since the seminal work conducted at Toyota Central Research Laboratories, polymer-clay nanocomposites have generated a lot of interest across industry. The unique physical properties of these nanocomposites have been explored by such varied industrial sectors as the automotive industry, the packaging industry, and plastics manufactures. These properties include improved mechanical properties, such as elastic modulus and tensile strength, thermal properties such as coefficient of linear thermal expansion and heat distortion temperature, barrier properties, such as oxygen and water vapor transmission rate, flammability resistance, ablation performance, solvent uptake, etc. Some of the related prior art is illustrated in U.S. Pat. Nos. 4,739,007; 4,810,734; 4,894,411; 5,102,948; 5,164,440; 5,164,460; 5,248,720; 5,854,326; 6,034,163; etc.
In general, the physical property enhancements for these nanocomposites are achieved with less than 20 vol. % addition, and usually less than 10 vol. % addition of the inorganic phase, which is typically clay or organically modified clay. Although these enhancements appear to be a general phenomenon related to the nanoscale dispersion of the inorganic phase, the degree of property enhancement is not universal for all polymers. It has been postulated that the property enhancement is very much dependent on the morphology and degree of dispersion of the inorganic phase in the polymeric matrix.
The clays in the polymer-clay nanocomposites are ideally thought to have three structures: (1) clay tactoids wherein the clay particles are in face-to-face aggregation with no organics inserted within the clay lattice; (2) intercalated clay wherein the clay lattice has been expanded to a thermodynamically defined equilibrium spacing due to the insertion of individual polymer chains, yet maintaining a long range order in the lattice; and (3) exfoliated clay wherein singular clay platelets are randomly suspended in the polymer, resulting from extensive penetration of the polymer into the clay lattice and its subsequent delamination. The greatest property enhancements of the polymer-clay nanocomposites are expected with the latter two structures mentioned herein above.
Attempts to incorporate clay in engineering thermoplastics have been made in the past. However, most common thermoplastic materials do not intercalate or exfoliate clay by itself. To overcome this problem, organically modified clays are developed which are basically clays intercalated with organic molecules which are further dispersed in the matrix thermoplastic. In a typical process, the clay is first intercalated with an organic molecule, such as a surfactant molecule, and subsequently the intercalated clay is added to the thermoplastic during melt-processing. A vast majority of intercalated clays are produced by interacting anionic clays with cationic surfactants including onium species such as ammonium (primary, secondary, tertiary, and quaternary), phosphonium, or sulfonium derivatives of aliphatic, aromatic or arylaliphatic amines, phosphines and sulfides. These onium ions can cause intercalation in the clay through ion exchange with the metal cations present in the clay lattice for charge balance. However, these surfactant molecules may degrade during melt-processing, placing severe limitation on the melt-processing temperature. Moreover, the surfactant intercalation is usually carried out in the presence of water, which needs to be removed by a subsequent drying step, adding to the cost of the end product.
Intercalation of clay with a polymer, as opposed to a low molecular weight surfactant, is also known in the art. There are two major intercalation approaches that are generally used—intercalation of a suitable monomer followed by polymerization (known as in-situ polymerization, see A. Okada et. Al., Polym Prep., Vol. 28, 447, 1987) or monomer/polymer intercalation from solution. Polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP) and polyethylene oxide (PEO) have been used to intercalate the clay platelets with marginal success. As described by Levy et.al, in “Interlayer adsorption of polyvinylpyrrolidone on montmorillonite”, Journal of Colloid and Interface Science, Vol 50 (3), 442, 1975, attempts were made to sorb PVP between the monoionic montmorillonite clay platelets by successive washes with absolute ethanol, and then attempting to sorb the PVP by contacting it with 1% PVP/ethanol/water solutions, with varying amounts of water. Only the Na-montmorillonite expanded beyond 20 Å basal spacing, after contacting with PVP/ethanol/water solution. The work by Greenland, “Adsorption of polyvinyl alcohol by montmorrilonite”, Journal of Colloid Science, Vol. 18, 647–664 (1963) discloses that sorption of PVA on the montmorrilonite was dependent on the concentration of PVA in the solution. It was found that sorption was effective only at polymer concentrations of the order of 1% by weight of the polymer. No further effort was made towards commercialization since it would be limited by the drying of the dilute intercalated layered materials. In a recent work by Richard Vaia et.al., “New Polymer Electrolyte Nanocomposites: Melt intercalation of polyethyleneoxide in mica type silicates”, Adv. Materials, 7(2), 154–156, 1995, PEO was intercalated into Na- montmorillonite and Li- montmorillonite by heating to 80° C. for 2–6 hours to achieve a d-spacing of 17.7° A. The extent of intercalation observed was identical to that obtained from solution (V. Mehrotra, E. P. Giannelis, Solid State Commun., 77, 155, 1991). Other, recent work (U.S. Pat. No. 5,804,613) has dealt with sorption of monomeric organic compound having at least one carbonyl functionality selected from a group consisting of carboxylic acids and salts thereof, polycarboxylic acids and salts thereof, aldehydes, ketones and mixtures thereof. Similarly U.S. Pat. No. 5,880,197 discusses the use of an intercalating monomer that contains an amine or amide functionality or mixtures thereof. In both these patents, and other patents issued to the same group, the intercalation is performed at very dilute clay concentrations in a medium such as water, leading to a necessary and costly drying step, prior to melt-processing.
Recently, polyether block polyamide copolymers have been identified to be a class of thermoplastic material with a wide range of specialty applications. One such family of copolymers has been developed by Elf Atochem as a product termed Pebax. The Pebax structure is described in the product literature to consist of a regular linear chain of rigid polyamide segments interspaced with flexible polyether segments. The various applications advertised for Pebax includes antistatic sheets or belts, food packaging materials, virus-proof surgical sheeting or garments, catheters, textiles as films for textile lamination for sports, leisure and workwear, footwear, gloves, water-resistant but breathable sheets, and many others. Some of these applications are disclosed in a number of patents such as U.S. Pat. Nos. 6,251,128; 6,221,042; 6,217,548; 6,203,920; 6,183,382; 6,133,375; 6,063,505; 5,951,494; 5,939,183; 5,807,796; 5,807,350; 5,662,975; 5,624,994; 5,614,588.
It is of great technical interest to further improve such a versatile material through the incorporation of nanoparticles, specifically layered materials such as clay, which can be intercalated and exfoliated in these copolymers without any further addenda. It is also of great interest to utilize these copolymers to intercalate and exfoliate nanoparticles such as clays and further disperse them in other matrix polymers of technical importance for enhancement of physical properties.
It is discovered that the aforementioned polyether block polyamide copolymers can easily intercalate clay by themselves during melt-processing, without requiring any other intercalating agent or solvent, forming the basis for nanocomposite materials of the instant invention.